News of the leak - which has only emerged as the result of an anonymous letter from within the plant - comes at an extraordinarily embarrassing time for the nuclear industry.
It follows the shutdown of operations at Dounreay, Britain's other nuclear reprocessing centre, in the north of Scotland for safety reasons, and comes as British Nuclear Fuels (BNFL), which runs Sellafield, is seeking permission to increase radioactive emissions from the site.
Yesterday a BNFL spokesman said: "It was not a leak." But he went on to say: "A pipe was perforated and fluid came out."
Environmentalists accused both BNFL and the official Nuclear Installations Inspectorate (NII) of "covering up" the leak, caused when a pipe carrying nuclear waste was perforated by a sharp metal shard. The plant "reprocesses" used nuclear fuel, separating it into plutonium, uranium and nuclear waste.
BNFL had referred publicly to "problems being experienced with the transfer of sheared finings" and the NII described it as "a technical problem relating to pipework wear".
The accident came to light because of an anonymous letter to Cumbrians Opposed to a Radioactive Environment (Core). It said "pipework has been found to be leaking" and that it could take months to repair.
The NII says: "In view of the present uncertainties over the mechanism which has led to the wear, and the need for us to be satisfied that the plant can be operated safely, we will not allow BNFL to restart the operation of this part of Thorp without our agreement."Reuse content